1. Field of the Invention
This invention generally relates to a stent for treating a disorder with a drug over a period of time extending beyond biodegradation of the stent scaffolding.
2. Description of the Background
In particular, the invention relates to radially expandable endoprostheses that are adapted to be implanted in a body lumen. An “endoprosthesis” corresponds to an artificial device that is placed inside the body. A “lumen” refers to a cavity of a tubular organ such as a body lumen. A stent is an example of such an endoprosthesis. Stents are generally cylindrically shaped devices which function to hold open and sometimes expand a segment of a body lumen or other anatomical lumen such as urinary tracts and bile ducts. Stents are often used in the treatment of atherosclerotic stenosis in body lumens. “Stenosis” refers to a narrowing or constriction of the diameter of a bodily passage or orifice. In such treatments, stents reinforce body vessels and prevent restenosis following angioplasty. “Restenosis” refers to the reoccurrence of stenosis in a body lumen or heart valve after it has been subjected to angioplasty or valvuloplasty.
The treatment of a diseased site or lesion with a stent involves both delivery and deployment of the stent. “Delivery” refers to introducing and transporting the stent through a body lumen to the treatment area in a body lumen. “Deployment” corresponds to the expanding of the stent within the lumen at the treatment area. Delivery and deployment of a stent are accomplished by positioning the stent at one end of a catheter, inserting the end of the catheter through the skin into a body lumen, advancing the catheter in the body lumen to a desired treatment location, expanding the stent at the treatment location, and removing the catheter from the lumen. In the case of a balloon expandable stent, the stent is mounted about a balloon disposed on the catheter. Mounting the stent typically involves compressing or crimping the stent onto the balloon. The stent is then expanded by inflating the balloon. The balloon may then be deflated and the catheter withdrawn. In the case of a self-expanding stent, the stent may be secured to the catheter via a retractable sheath or a sock.
When the stent is in a desired bodily location, the sheath may be withdrawn allowing the stent to self-expand. This requires a sufficient degree of strength and rigidity or stiffness. In addition to having adequate radial strength, the stent should be longitudinally flexible to allow it to be maneuvered through a tortuous vascular path.
Thus, a stent is typically composed of a scaffolding that includes a pattern or network of interconnecting structural elements or struts. The scaffolding can be formed of tubes, or sheets of material rolled into a cylindrical shape. The scaffolding is designed to allow the stent to be radially expandable. The pattern is generally designed to maintain the longitudinal flexibility and radial rigidity required of the stent. Longitudinal flexibility facilitates delivery of the stent and radial rigidity is needed to hold open a body lumen. A medicated stent may be fabricated by coating the surface of either a metallic or polymeric scaffolding with a polymeric carrier that includes a bioactive agent. The polymeric scaffolding may also serve as a carrier of bioactive agent.
In many treatment applications of stents, the presence of a stent in a body may be necessary for a limited period of time until its intended function of, for example, maintaining vascular patency and/or drug delivery is accomplished. Thus, stents are often fabricated from biodegradable, bioabsorbable, and/or bioerodable materials such that they completely erode only after the clinical need for them has ended. In addition, a stent should also be capable of satisfying the mechanical requirements discussed above during the desired treatment time.
A polymeric stent should be mechanically stable throughout the range of stress experienced during use. In addition to mechanical stability, a stent should have a sufficient rate of biodegradability or erosion as dictated by a treatment regimen. However, one of the major clinical challenges of bioabsorbable stents is adequately suppressing inflammatory responses triggered by the degradation of the stent. The embodiments of the invention address this and other concerns.